1. Field of the Invention
The present invention relates to a secondary battery pack incorporating a secondary battery such as a lithium ion battery or the like.
2. Background of the Invention
A secondary battery which is a rechargeable battery may be housed in a casing called a battery pack. This battery pack houses secondary batteries of a predetermined number which are connected to one another in series or in parallel (or houses only one secondary battery), and incorporates a protection circuit for the secondary batteries housed therein to prevent the secondary batteries from being overcharged or overdischarged and to prevent the second batteries from being damaged because of short circuit of terminal portions. The secondary battery pack sometimes incorporates circuits required when the secondary batteries are charged, circuits for adjusting a charging voltage and a charging current, and so on.
FIG. 1 is a diagram showing an example of a secondary battery pack incorporating a charging circuit and a protection circuit. This secondary battery pack has a positive side terminal portion 1 and a negative side terminal portion 2. The positive side terminal portion 1 is connected to a positive electrode of a secondary battery B through a charging circuit 3 and a protecting circuit 4. A negative electrode of the secondary battery B is connected to the negative side terminal portion 2. The charging circuit 3 is a circuit for, under the control of a charge control circuit 5, adjusting a voltage and a current obtained from the outside to supply the same to the secondary battery B. The protecting circuit 4 is a circuit for separating the secondary battery B from the terminal 1 side when the protection control circuit 6 detects an abnormal state of a voltage between terminals of the secondary battery B or the like. While in this embodiment the charging circuit 3 and the protecting circuit 4 are connected between the positive side terminal portion 1 and the secondary battery B, they may be connected between the negative side terminal portion 2 and the secondary battery B.
FIG. 2 is a diagram showing an example of the charging circuit 3 and a control circuit 5 therefor. In FIG. 2, a lithium ion battery is employed as the secondary battery B. When the lithium ion battery is employed, a constant current charging characteristic and a constant voltage charging characteristic must be considered upon a charging operation. A current detecting circuit 5a detects a charging current obtained at the terminal portion 1 to control a charging current flowing from a constant current circuit 3a connected between the terminal portion 1 and the secondary battery B toward the secondary battery B side. A voltage detecting circuit 5b detects the voltage between the terminals of the secondary battery B to control a voltage applied from a constant voltage circuit 3b connected between the terminal portion 1 and the secondary battery B toward the secondary battery b side to be a predetermined constant voltage.
FIG. 3 is a diagram showing an example of the protection circuit 4 and a control circuit 6 therefor. A detecting circuit 6a detects a voltage difference between both ends of a resistor R0 connected to one electrode of the secondary battery B, thereby detects a current flowing through the secondary battery B as a voltage. Based on a signal indicative of detection of the voltage, a switch circuit 4a connected between the secondary battery B and the negative side terminal portion 2 is controlled.
While in this example the protecting circuit for detecting a current is described, a protecting circuit controlled by detection of a voltage such as a battery voltage or the like may be employed.
Since the charging circuit and the protecting circuit are formed as separate circuits as described above, an arrangement of a circuit incorporated in a secondary battery pack disadvantageously becomes complicated. Specifically, since a voltage detected by a voltage detecting circuit for controlling a charging operation (e.g., a voltage used for a charging operation with a constant voltage) is different from a voltage detected by a protecting circuit for protecting a secondary battery and the like (e.g., a maximum voltage applicable to a secondary battery), the voltages of two kinds must independently be detected by the charging circuit and the protecting circuit and controlled, which leads to a complicated circuit arrangement of the secondary battery pack.